It depends on your relationship to them, but in no case would it be acceptable to say "Greetings, teachers" unless you are able to carry off being very sardonic.

"Hello" is a good generic greeting, acceptable for almost any kind of relationship.

"Good morning/afternoon" is a little more formal.

"Greetings ladies/gents/ladies and gentlemen" is either very formal, or rather informal and a little facetious. I doubt this will be appropriate for your relationship with them unless you are friends or work colleagues who are in at least semi-regular contact.

You can say 'Hi, guys!' and nod and smile at them. That is most common.

Since 'guy' is originally a term for a male, older women can get ruffled at being addressed as 'guys' if there are no men in their group. However moderately aged and younger women won't care because it is by far the most common way to address a group of people informally.

In the case of a group of all older women, you can say 'You all'. Such as 'How are you all doing today?' In the case of a simple greeting, it is more normal sounding to leave off the 'you all' and just say 'Hello' and pass.

NEVER say the word 'ladies' at the end of a sentence, such as 'Hello, ladies!'. It automatically adds a creeper vibe (meaning you will sound creepy and women will avoid you) to whatever you are saying.

A common american would say 'Hey, whats up' or just 'what up girls' but that is extremely informal and would only be used on a group of women your own age.